U.S. patent application number 11/732756 was filed with the patent office on 2008-10-09 for method and apparatus for measuring the output current of a switching regulator.
Invention is credited to Eric Smith.
Application Number | 20080246460 11/732756 |
Document ID | / |
Family ID | 39826390 |
Filed Date | 2008-10-09 |
United States Patent
Application |
20080246460 |
Kind Code |
A1 |
Smith; Eric |
October 9, 2008 |
Method and apparatus for measuring the output current of a
switching regulator
Abstract
One embodiment of the present invention provides an apparatus
that measures the average-output-current produced by a switching
regulator within an electronic device. The apparatus includes
current-sensing-circuitry coupled to a switching
field-effect-transistor (FET) within the switching regulator,
wherein the current-sensing-circuitry is configured to bypass a
small sense current from the conducting current of the
switching-FET according to a sense ratio, wherein the conducting
current is controlled by a control signal for the switching
regulator. The apparatus also includes a
current-to-voltage-converter coupled to the
current-sensing-circuitry which is configured to convert the sense
current into a sense voltage. The apparatus further includes
voltage-averaging-circuitry which is configured to produce an
average-sense-voltage from the sense voltage. This sense voltage is
coupled to the input of the voltage-average-circuitry through a
switch, which is gated by the control signal. The
average-output-current of the switching regulator is indicated by
the average-sense-voltage.
Inventors: |
Smith; Eric; (San Jose,
CA) |
Correspondence
Address: |
PVF -- APPLE INC.;c/o PARK, VAUGHAN & FLEMING LLP
2820 FIFTH STREET
DAVIS
CA
95618-7759
US
|
Family ID: |
39826390 |
Appl. No.: |
11/732756 |
Filed: |
April 3, 2007 |
Current U.S.
Class: |
324/76.11 |
Current CPC
Class: |
G01R 19/0092 20130101;
G01R 19/003 20130101 |
Class at
Publication: |
324/76.11 |
International
Class: |
G01R 19/00 20060101
G01R019/00 |
Claims
1. An apparatus that measures an average-output-current produced by
a switching regulator within an electronic device, comprising:
current-sensing-circuitry coupled to a switching
field-effect-transistor (FET) within the switching regulator,
wherein the current-sensing-circuitry is configured to bypass a
small sense current from the conducting current of the
switching-FET according to a sense ratio, wherein the conducting
current is controlled by a control signal for the switching
regulator; a current-to-voltage-converter coupled to the
current-sensing-circuitry configured to convert the sense current
into a sense voltage; and voltage-averaging-circuitry configured to
produce an average-sense-voltage from the sense voltage, wherein
the sense voltage is coupled to the input of the
voltage-average-circuitry through a switch; and wherein the switch
is gated by the control signal; wherein the average-output-current
of the switching regulator is indicated by the
average-sense-voltage.
2. The apparatus of claim 1, wherein the current-sensing-circuitry
includes a sense FET coupled in parallel with a low-side switching
FET in the switching regulator.
3. The apparatus of claim 2, wherein the current-to-voltage
converter is configured to provide both the sense FET and the
low-side switching FET with the same voltage.
4. The apparatus of claim 1, wherein the
voltage-averaging-circuitry includes a track-and-hold circuit.
5. The apparatus of claim 1, wherein the voltage-average-circuitry
is configured to average the sense voltage input when the switch is
activated by the control signal and is configured to maintain the
average-sense-voltage when the switch is deactivated by the control
signal, thereby facilitating tracking of the average-sense-voltage
during a period of the control signal.
6. The apparatus of claim 1, wherein the average-output-current is
used to determine power consumption of the electronic device.
7. The apparatus of claim 1, wherein the current-sensing-circuitry,
the current-to-voltage-converter, and the
voltage-averaging-circuitry are integrated onto an
integrated-circuit chip.
8. The apparatus of claim 1, wherein the
current-to-voltage-converter and the voltage-averaging-circuitry
are integrated onto an integrated-circuit chip.
9. A method for measuring an average-output-current produced by a
switching regulator within an electronic device, comprising: using
current-sensing-circuitry which is coupled to a switching
field-effect-transistor (FET) within the switching regulator to
bypass a small sense current from the conducting current of the
switching-FET according to a sense ratio, wherein the conducting
current is controlled by a control signal for the switching
regulator; converting the sense current into a sense voltage using
a current-to-voltage-converter coupled to the
current-sensing-circuitry; and producing an average-sense-voltage
from the sense voltage using voltage-averaging-circuitry, wherein
the sense voltage is coupled to the input of the
voltage-average-circuitry through a switch; and wherein the switch
is gated by the control signal; wherein the average-output-current
of the switching regulator is indicated by the
average-sense-voltage.
10. The method of claim 9, wherein the current-sensing-circuitry
includes a sense FET coupled in parallel with a low-side switching
FET in the switching regulator.
11. The method of claim 10, wherein the current-to-voltage
converter is configured to provide both the sense FET and the
low-side switching FET with the same voltage.
12. The method of claim 9, wherein the voltage-averaging-circuitry
includes a track-and-hold circuit.
13. The method of claim 9, further comprising: averaging the sense
voltage input using the voltage-average-circuitry when the switch
is activated by the control signal; and maintaining the
average-sense-voltage using the voltage-average-circuitry when the
switch is deactivated by the control signal, thereby facilitating
tracking of the average-sense-voltage during a period of the
control signal.
14. The method of claim 9, further comprising determining a power
consumption of the electronic device based on the
average-output-current.
15. The method of claim 9, wherein the current-sensing-circuitry,
the current-to-voltage-converter, and the
voltage-averaging-circuitry are integrated onto an
integrated-circuit chip.
16. The method of claim 9, wherein the current-to-voltage-converter
and the voltage-averaging-circuitry are integrated onto an
integrated-circuit chip.
Description
BACKGROUND
[0001] 1. Field of the Invention
[0002] The present invention relates to techniques for determining
the power consumption of an electronic device, such as an
integrated circuit (IC) chip. More specifically, the present
invention relates to a method and apparatus for determining the
power consumption of an electronic device by measuring an average
output current generated by a switching regulator that supplies
power to the electronic device.
[0003] 2. Related Art
[0004] Rapid advances in computing technology presently make it
possible to perform trillions of operations each second on data
sets as large as a trillion bytes. These advances can be largely
attributed to an exponential increase in the density and complexity
of integrated circuits (ICs). Unfortunately, in conjunction with
this increase in computational power, power consumption and heat
dissipation of ICs has also increased dramatically.
[0005] Increasing power consumption and associated heat dissipation
creates serious challenges for power management and cooling in
computing devices, especially for portable computers. If the
real-time power consumption of system components can be measured,
the system can provision power to system components more
intelligently, and can also adjust cooling mechanisms, for example,
by increasing/decreasing fan speed, to more efficiently remove
waste heat from the computer system.
[0006] Modern power supplies within computer systems often utilize
switching regulators to provide a substantially constant voltage to
drive system components, such as IC chips. A switching regulator
typically comprises control logic, a switching circuit, and an "LC
tank" circuit. The control logic typically generates two
square-wave control signals that are complements of each other. The
switching circuit typically includes at least one high-side
switching metal-oxide-semiconductor field-effect transistor
(MOSFET) and one low-side switching MOSFET, which are coupled in
series. The two out-of-phase control signals from the control logic
are coupled to the gates of the two switching MOSFETs to drive the
two MOSFETs. Because each control signal switches between a high
voltage and a low voltage, the two MOSFETs will be turned on and
off periodically by the control signals.
[0007] To convert AC output currents from the MOSFETs into a DC
voltage, the MOSFETs are coupled to the LC tank circuit.
Specifically, when the high-side MOSFET is turned on and the
low-side MOSFET is turned off, the output current flows through the
inductor L and capacitor C. This causes energy to be stored within
the inductor and the capacitor. A portion of the output current
also drives the load. Next, when the high-side MOSFET is turned off
and the low-side MOSFET is turned on, the energy stored within the
inductor and the capacitor continues to provide a near DC drive
current to the load.
[0008] Note that the output current from the switching regulator to
the load can change dynamically during system operation. For
example, the output current to a CPU typically increases as the
utilization of the CPU increases, whereas the output current to the
CPU decreases as the CPU utilization drops. During this time, the
voltage on the load remains constant. Consequently, one can monitor
the power usage of the load by monitoring the average output
current from the switching regulator.
[0009] One technique to measure the average output current from the
switching regulator is to insert a current sensing component, for
example, an ammeter, in series with the load. However, this
technique can cause significant amount of dissipative loss on the
current sensor because the entire output current flows through this
current-sensing component.
[0010] Another technique is to use a component that is already in
series with the load to measure the output current. For example,
one can obtain the output current by first measuring the voltage
across inductor L and then computing the current by dividing the
voltage by the resistance of inductor L. Unfortunately, the
resistance of inductor L is typically not a constant and is
difficult to measure. For example, this resistance can change
significantly because of temperature variations.
[0011] Hence, what is needed is a method and an apparatus for
determining the average output current from a switching regulator
without the problems described above.
SUMMARY
[0012] One embodiment of the present invention provides an
apparatus that measures the average-output-current produced by a
switching regulator within an electronic device. The apparatus
includes current-sensing-circuitry coupled to a switching
field-effect-transistor (FET) within the switching regulator,
wherein the current-sensing-circuitry is configured to bypass a
small sense current from the conducting current of the
switching-FET according to a sense ratio, wherein the conducting
current is controlled by a control signal for the switching
regulator. The apparatus also includes a
current-to-voltage-converter coupled to the
current-sensing-circuitry which is configured to convert the sense
current into a sense voltage. The apparatus further includes
voltage-averaging-circuitry which is configured to produce an
average-sense-voltage from the sense voltage. This sense voltage is
coupled to the input of the voltage-average-circuitry through a
switch, which is gated by the control signal. The
average-output-current of the switching regulator is indicated by
the average-sense-voltage.
[0013] In a variation on this embodiment, the
current-sensing-circuitry includes a sense FET coupled in parallel
with a low-side switching FET in the switching regulator.
[0014] In a further variation on this embodiment, the
current-to-voltage converter is configured to provide both the
sense FET and the low-side switching FET with the same voltage.
[0015] In a variation on this embodiment, the
voltage-averaging-circuitry includes a track-and-hold circuit.
[0016] In a variation on this embodiment, the
voltage-average-circuitry is configured to average the sense
voltage input when the switch is activated by the control signal
and is configured to maintain the average-sense-voltage when the
switch is deactivated by the control signal, thereby facilitating
tracking of the average-sense-voltage during a period of the
control signal.
[0017] In a variation on this embodiment, the
average-output-current is used to determine power consumption of
the electronic device.
[0018] In a variation on this embodiment, the
current-to-voltage-converter and the voltage-averaging-circuitry
are integrated onto an IC chip. The current-sensing-circuitry can
also be integrated onto the IC chip.
BRIEF DESCRIPTION OF THE FIGURES
[0019] FIG. 1A presents an exemplary circuit diagram of a typical
switching regulator.
[0020] FIG. 1B illustrates inductor current as a function of time
during switching operation in accordance with an embodiment of the
present invention.
[0021] FIG. 2 illustrates an exemplary circuit diagram for a
switching regulator coupled to average-output-current-measurement
circuitry in accordance with an embodiment of the present
invention.
[0022] FIG. 3 illustrates an integrated average-current-measurement
circuit 300 in accordance with an embodiment of the present
invention.
DETAILED DESCRIPTION
[0023] The following description is presented to enable any person
skilled in the art to make and use the invention, and is provided
in the context of a particular application and its requirements.
Various modifications to the disclosed embodiments will be readily
apparent to those skilled in the art, and the general principles
defined herein may be applied to other embodiments and applications
without departing from the spirit and scope of the present
invention. Thus, the present invention is not limited to the
embodiments shown, but is to be accorded the widest scope
consistent with the principles and features disclosed herein.
Overview
[0024] The present invention determines the real-time power
consumption of an integrated circuit (IC) chip by measuring the
average output current from the switching regulator that supplies
power to the IC chip. Specifically, a current-sensing-circuit is
used to bypass a tiny sense current from a main
switching-regulator-output-current according to a precise ratio. A
current-to-voltage-converter is then used to convert this sense
current into a sense voltage, which can be more conveniently
manipulated. Next, the sense voltage is averaged using a
voltage-averaging-circuit to produce an average readout. In
particular, the voltage-averaging operation is synchronized with
the switching operation of the switching regulator, thereby
allowing the average readout to be an accurate representation of
the average output current of the switching regulator.
A Typical Switching Regulator
[0025] FIG. 1A presents an exemplary circuit diagram of a typical
switching regulator 100. It contains a high-side switching
metal-oxide-semiconductor field-effect-transistor MOSFET 102
("high-side FET" hereafter) and a low-side switching MOSFET 104
("low-side FET" hereafter), which are both controlled by control
logic 106. In one embodiment of the present invention, MOSFET 102
and 104 are power MOSFETs.
[0026] Control logic 106 drives the FETs 102 and 104 separately
using periodic control signals 108 and 110 (shown as squares
waves), wherein the two control signals are complements of each
other. Hence, during each control period, the two switching FETs
take turns conducting current.
[0027] More specifically, when high-side FET 102 is turned on by
control signal 108 (i.e., when control signal 108 is high and
control signal 110 is low), the drain voltage supply "V.sub.1"
causes a linearly increasing current I.sub.L to flow through
inductor 112 and capacitor 114. This allows energy to be stored in
the inductor and the capacitor. Part of the conducting current also
drives load 118. Meanwhile, low-side FET 104 is inactive.
[0028] When low-side FET 104 is conducting (i.e., when control
signal 110 is high and control signal 108 is low), both inductor
112 and capacitor 114 release stored energy to drive load 118,
which causes the inductor current I.sub.L to decease linearly. Note
that during each switching cycle, capacitor 114 operates to "smooth
out" the ripples in the voltage at node 116 so that load 118
receives a "regulated" voltage. Meanwhile, high-side FET 102 is
inactive.
[0029] FIG. 1B illustrates inductor current as a function of time
during switching operation in accordance with an embodiment of the
present invention. Note that inductor current I.sub.L has a
sawtooth waveform wherein the shape and amplitude of the sawtooth
is determined by the duty cycle of control signals 108 and 110.
Also note that inductor current I.sub.L has an average value
I.sub.ave which remains at substantially the same value during the
two phases of each switching cycle. Consequently, one can measure
the average switching regulator current when either the high-side
FET is active or the low-side FET is active.
Measuring Average Output Current of the Switching Regulator
[0030] FIG. 2 illustrates an exemplary circuit diagram for a
switching regulator coupled to average-output-current-measurement
circuitry in accordance with an embodiment of the present
invention.
[0031] In one embodiment of the present invention, the
average-output-current-measurement circuitry includes three
subcomponents: a current sensing circuit, a current-to-voltage
converter, and a voltage-averaging circuit. We describe each of
subcomponents in more detail below.
[0032] Current Sensing Circuit
[0033] The embodiment of the present invention illustrated in FIG.
2 utilizes a current sensing circuit to bypass a tiny sense current
from the main current being measured. In this embodiment, a current
sensing circuit 202 comprises a sensing FET 204 which is coupled in
parallel with a low-side FET 206. More specifically, the drains and
gates of both sensing FET 204 and low-side FET 206 are tied
together. Hence, the same control signal that controls low-side FET
206 also controls sensing FET 204. The source of low-side FET 206
is connected to the ground while the source of sensing FET 204 is
used as the output node. Although the sources of the two FETs are
not tied together, it is desirable that they have the same voltage.
We describe a technique that sets the source voltage of sensing FET
204 to be the same as the source voltage of low-side FET 206
below.
[0034] Sensing FET 204 conducts a tiny sense current 208, which is
a small fraction of a main switching regulator current 209
conducted by low-side FET 206. The ratio between main switching
regulator current 209 and sense current 208 is proportionate to a
predetermined large number. In one embodiment, this ratio is
greater than 500.
[0035] In one embodiment of the present invention, sensing FET 204
and low-side FET 206 are fabricated using the same semiconductor
processes and same materials, and therefore the sense ratio can be
precisely controlled by the physical design parameters, such as by
the gate width to gate length ratio (W/L). Note that if the
gate-to-source voltage V.sub.GS and drain-to-source voltage
V.sub.DS are substantially the same for these two FETs, the sense
ratio equals (W/L).sub.FET 206/(W/L).sub.FET 204. Note that both
FETs 204 and 206 can be N-type FETs or P-type FETs. Also note that
the drain and the source within each FET is interchangeable.
[0036] Although we describe a current sensing circuit 202 in the
context of a simple sensing FET, other techniques can be used. For
example, it is possible to bypass a small sense current according
to a precise ratio from the main current without using sensing FET
204. In one embodiment of the present invention, current sensing
circuit 202 can be alternatively coupled in parallel with high-side
FET 210 to draw a sense circuit when high-side FET 210 is
conducting.
[0037] Note that because sense current 208 is significantly smaller
than the main current, sensing circuit 208 consumes very little
power and has negligible effect on the switching regulator.
However, this current can be difficult to measure because it has an
AC behavior and very small amplitude. This problem can be remedied
by applying a gain to the sensing current as is described
below.
[0038] Current Gain Stage
[0039] Sense current 208 in sensing FET 204 is very small and
therefore can be difficult to measure. Hence, one embodiment of the
present invention provides a gain to the sense current prior to
measuring this current. More specifically, this gain can be
provided by a current-to-voltage (I-V) converter 212. Generally, an
I-V converter converts a current i to a voltage v according to
v=iR, wherein R is a known resistance. As seen in FIG. 2, I-V
converter 212 comprises: an operational amplifier (op-amp) 214, a
high precision resistor 216 which is coupled to a voltage source
"V.sub.2", and a transistor 218 which is coupled between resistor
216 and the output of op-amp 214. Note that resistor 216 can also
be implemented as an active load.
[0040] Inverting input 220 of op-amp 214 is coupled to the source
node of sensing FET 204. Additionally, non-inverting input 222 of
op-amp 214 is connected to the ground, which also brings the
voltage at inverting input 220 and the source node of sensing FET
204 to ground (due to a "virtual ground"). Consequently, low-side
FET 206 and sensing FET 204 have exactly the same voltages V.sub.GS
and V.sub.DS, which ensures that the current ratio between the two
FETs is based on the predetermined sense ratio.
[0041] Note that because op-amp 214 and voltage V.sub.2 do not draw
current from the inputs, the current flowing through resistor 216
and transistor 218 also equals sense current 208. Hence, the drain
voltage of transistor 218 equals V.sub.2-I.sub.senseR, wherein
I.sub.sense represents sense current 208. Because V.sub.2 and R are
both known, this drain voltage can be used as an accurate indicator
of sense current 208. We refer to this drain voltage as sense
voltage 224 below.
[0042] Note that the current gain stage in the present invention is
not limited to the specific configuration of I-V converter 212. Any
other circuit that is capable of holding the source node of sensing
FET 204 to ground and performing a current-to-voltage conversion
can be used in place of I-V converter 212.
[0043] Voltage Averaging Circuit
[0044] Sense voltage 224 is still an AC signal, but is considerably
easier to manipulate than sense current 208. As seen in FIG. 2,
sense voltage 224 is coupled to a voltage-averaging-circuit 226,
which performs an averaging operation on this input voltage.
[0045] In one embodiment of the present invention,
voltage-averaging-circuit 226 is a track-and-hold (T/H) circuit.
This T/H circuit comprises a switch 228 and a capacitor 230. In one
embodiment of the present invention, switch 228 is implemented
using a MOSFET 228. As seen in FIG. 2, the gate of MOSFET 228 is
coupled to the control signal which controls both low-side FET 206
and sensing FET 204. Meanwhile, the drain voltage of MOSFET 228 is
coupled to sense voltage 224.
[0046] The averaging operation in FIG. 2 proceeds as follows.
[0047] Tracking Phase: When the control signal is high, MOSFET 228
is turned on. This allows sense voltage 224 to charge or discharge
capacitor 230, depending on the previous voltage value at node 232.
The charging/discharging process "smoothes out" the input waveform
and results in a near constant voltage V on node 232. Note that
this average process is synchronized with the time window when
low-side FET 206 is conducting. More specifically, it begins when
FET 206 turns on and ends when FET 206 turns off. Consequently, the
average voltage V at node 232 provides an accurate representation
of sense current 208. [0048] Holding Phase: When the control signal
is low, MOSFET 228 is turned off. Because there is no current path
for capacitor 230, capacitor 230 holds the average voltage V at
node 232 until next control signal period begins.
[0049] Note that the average voltage V is valid during a full
control logic period. This is possible because the control signal
for the low-side FET is also used to synchronize the current
sensing at sensing FET 204, and the voltage averaging at node 232.
In one embodiment of the present invention, the average voltage V
at node 232 can be accurately measured by inserting a voltage
follower 234 which decouples the capacitor 226 from the voltage
measuring mechanism at V.sub.out 236.
[0050] Because the average output voltage V can be measured, an
average sense current .sub.sense is obtained according to (V.sub.2-
V)/R. Consequently, the average current produced by the switching
regulator during each control period equals N(V.sub.2- V)/R,
wherein N is the sense ratio. Furthermore, the power consumed by
the load can be obtained by multiplying this average current by the
constant voltage on the load.
[0051] Note that although we describe using a simple MOSFET and a
capacitor to perform a timed voltage averaging operation, other
circuits that perform a timing controlled voltage averaging
function can be used in place of voltage averaging circuit 226.
[0052] Integrated Circuit
[0053] Note that the above described three circuit modules can be
integrated into a single IC module.
[0054] For example, FIG. 3 illustrates an integrated
average-current-measurement circuit 300 in accordance with an
embodiment of the present invention. Specifically,
average-current-measurement circuit 300 includes sensing FET 302,
I-V converter 304, and voltage-averaging-circuit 306, which are
coupled together in the same manner as described in FIG. 2. IC 300
further comprises an input 308 for receiving the control signal for
sensing FET 302 and circuit 306, and an input 310 which is coupled
to the drain of low-side FET of the switching regulator. IC 300
also comprises a single voltage output 312.
[0055] In a further embodiment, the low-side FET and the sensing
FET can be replaced by an integrated current sensing FET, which
comprises a main power FET and a sense FET. In this embodiment, the
I-V converter and the voltage-averaging-circuit can be further
integrated into an IC chip. This integrated IC has a number of
inputs, which include a current input from the sense FET, and a
control input from the control logic. The IC provides a single
voltage output which represents the average switching regulator
current.
[0056] Note that in both embodiments, the integrated ICs can be
disabled by the control signal input.
[0057] The foregoing descriptions of embodiments of the present
invention have been presented only for purposes of illustration and
description. They are not intended to be exhaustive or to limit the
present invention to the forms disclosed. Accordingly, many
modifications and variations will be apparent to practitioners
skilled in the art. Additionally, the above disclosure is not
intended to limit the present invention. The scope of the present
invention is defined by the appended claims.
* * * * *